The present invention relates to methods of forming precision metal parts and, more specifically, to thixotropic forming of precision multi-alloy parts.
As performance criteria for turbine engines becomes more stringent, there is a need for an improved turbine rotor that exhibits maximum resistance to both fatigue and creep.
Die casting is a well-known process for producing complex components with excellent surface quality and good dimensional accuracy. However, the structural integrity of die castings is often compromised by air trapped in the casting upon injection of the liquid metal into the die casting cavity. The resultant porosity also compromises heat treatment of the casting which is often necessary to refine the grain structure and increase the strength of the casting.
Forging is also a well known process for producing relatively strong components having a desirable grain structure. However, forged products generally exhibit relatively low resistance to creep.
Thixotropic, or semisolid, metal forming is a viable alternative to traditional casting and forging methods. This process lies somewhere between a casting and a forging process in that the slug of metal to be formed will be brought to a "thixotropic" phase; that is, 30 or 40 percent of the mass will be in a liquid phase and the balance in a solid phase. The solid portion comprises small spherically-shaped nodules suspended within the liquid phase. Semisolid metals heated to a thixotropic phase exhibit unique Theological properties due to their non-dendritic, or spherical, microstructure. The rheological properties of the semisolid metal range from high viscosities, like table butter, for alloys at rest, to low viscosities, such as machine oil, as the shearing rate of the semisolid slug is increased. By heating the metals to a semisolid range and then agitating the semisolid alloy, the dendritic microstructure normally found is eliminated and replaced by the spherical microstructure. Upon solidification, the alloys then exhibit a fine equiaxed microstructure.
Normally, a highly viscous thixotropic slug will retain its outer shape provided there are no external forces, other than gravity, applied to it. However, its butter-like consistency is easily deformed to a low viscosity, particularly by a shearing action such as high velocity impact, making it extremely suitable when driving the alloy into the mold during the manufacturing process. Because semisolid-formed alloys exhibit an intermediate-sized grain structure, larger than forged grains and smaller than cast grains, it is expected that semisolid forged or cast alloys will have improved creep rupture resistance over traditionally forged alloys and improved strength properties over traditionally cast alloys.
The thixotropic process has been extensively studied by others in relation to lighter metals such as aluminum, magnesium, zinc, and copper alloys. However, very little research has occurred with regard to high temperature alloys commonly used in turbine rotors, including ferrous or nickel-based alloys. One significant difference between semisolid production for lighter alloys and that for high temperature alloys involves the adaptation of the process to the problematic and high heating temperatures of 2500.degree. F. to 2700.degree. F. as opposed to alloys in the 1200.degree. F. melting point range. Designing a semisolid process compatible with such high heat has proven challenging. Generally, chrome-nickel alloys of, for example, 18% Cr and 82% Ni are used in turbine rotor forgings. This alloy has a solidus of 2550.degree. F., and a liquidus of 2640.degree. F. where the alloy is completely molten. The semisolid/thixotropic phase exists between the solidus and liquidus temperatures at temperatures ranging between 2550.degree. F. and 2640.degree. F. The alloy is commonly forged at temperatures below 2550.degree. F., in the solid phase, and cast at molten temperatures above 2640.degree. F., in the liquid phase.
Yet another problem that must be addressed is that current forging and casting equipment design includes permanent molds that often do not readily separate from the part interface when removing the turbine rotors and their intricate blades from the mold. This results in fractured or weakened blades and a corresponding number of rejected parts that do not meet design specifications. A need exists for semisolid manufacturing methods that facilitate ease of removal of the finished part, thereby improving the production volume and reducing the rejection rate of the finished parts.
Finally, precision metal assemblies are specifically designed to withstand various forces under uniquely stressful conditions. In certain applications, however, one part of a complete assembly may be exposed to stress and temperature loads significantly different from that of other parts integral to the same assembly. For example, the bore of a rotor may require good elongation, high strength, and good low cycle fatigue properties but may not require high temperature properties. In contrast, certain blade or rim portions of the rotor might require very high creep resistance and stress rupture strength at elevated temperatures. Formulating a single alloy capable of withstanding the variable stresses subjected to different locations within a precision metal assembly has also proven challenging. Therefore, a need exists for semisolid manufacturing methods that can be modified to vary the properties of different parts integral to a complete assembly.